Grounding terminal and mounting structure of the same on a printed circuit board

ABSTRACT

The present invention realizes proper grounding of a printed circuit board to a grounding conductor by solving the problems in soldering a grounding terminal to a printed circuit board by reflow soldering, relating to a mounting structure of a grounding terminal on a printed circuit board formed by soldering a grounding terminal to the printed circuit board. Two joint surfaces having different surface areas are formed within the joint part of the grounding terminal, and the grounding terminal is soldered to the conductive pattern on the printed circuit board corresponding to the joint surfaces by using the solder having an amount corresponding to the surface area of each joint surface. Also, the bending area as the center of elastic deformation of the contact part of the grounding terminal is formed at a predetermined distance away from the relatively small joint surface of the joint part.

This application is a division of application Ser. No. 09/301,807, filedApr. 28, 1999, now U.S. Pat. No. 6,300,579.

FIELD OF THE INVENTION

The invention relates to a mounting structure of a grounding terminal ona printed circuit board, wherein the printed circuit board is groundedto a grounding conductor by the grounding terminal, which is mounted onthe surface of the printed circuit board by reflow soldering, contactingwith and being pressed against the grounding conductor.

BACKGROUND OF THE INVENTION

Conventionally, a mounting structure of a grounding terminal on aprinted circuit board for grounding the printed circuit board, in whicha grounding terminal is mounted on the surface of the printed circuitboard and pressed against a grounding conductor, is known. FIG. 5 is aplan view of such a conventional grounding structure and FIG. 6 is alongitudinal sectional view along line VI—VI of FIG. 5. As shown in FIG.6, the grounding terminal 100 is soldered to the conductive pattern 201on the printed circuit board 200 with solder 202. Then, the groundingterminal 100 is pressed against the grounding conductor, therebygrounding the printed circuit board 200. To illustrate the structuresimply, the grounding conductor 300 is not shown in FIG. 5.

The grounding terminal 100 is formed by bending a strip-like thin sheetmetal member with a width of α, for example, 0.8 mm, and has a jointpart 101 with a longitudinal length of β, for example, 3.5 mm. Onesurface of the joint part 101 is a joint surface 101 a which is solderedto the conductive pattern 201 on the printed circuit board 200. Also,the grounding terminal has a contact part 102 which adjoins the jointpart 101 and is bent in the upper direction. An outer surface 102 a ofthe contact part 102 is pressed against the grounding conductor 300. Asthe result of load by the grounding conductor, the contact part 102 iselastically deformed around the bending area γ, and is pressed againstthe grounding conductor 300 by restoring force caused by the elasticdeformation, thereby electrically connecting the printed circuit board200 and the grounding conductor 300 properly.

When the aforementioned grounding terminal is soldered to the printedcircuit board, reflow soldering is usually employed. Reflow solderingcomprises the steps of previously applying a suitable amount of solderto the point to be jointed, fixing the grounding terminal to the pointtemporarily, and melting the solder by using an external heat source,and all these steps are performed automatically. An advantage of reflowsoldering is that by allowing a suitable amount of solder to be applied,it prevents solder from being placed at inappropriate points of theprinted circuit board, and thus prevents improper soldering such as adefective bridge. Also, there is another advantage that in the casewhere a relatively small mounting member as aforementioned is to bemounted, self-alignment effect can be expected. That is, even if thetemporarily fixed member deviates a little from the conductive patternof the printed circuit board, the member is moved to the right positionby surface tension of the melted solder.

However, there are the following problems in reflow soldering of theabove conventional grounding terminal:

(1) The grounding terminal 100 is moved to the right position byself-alignment effect by surface tension of the melted solder, and thenis finally positioned by hardening of the melted solder. During thehardening process of the solder, the grounding terminal 100 sometimesdeviates from the right position on the printed circuit board 200because of, for example, vibration. This results from the fact thatsince the amount of the solder is large, the thermal capacity of thesolder is large, and thus it takes time for the solder to harden. If thegrounding terminal 100 deviates from the right position, the contactpart 102 may not be pressed against the grounding conductor 300 in anappropriate state. Furthermore, the requirements of highly precisemounting performance for allowing high density mounting are notsatisfied.

(2) When the temperature of the grounding terminal 100 reaches themelting point of the solder earlier than that of the conductive pattern201 of the printed circuit board 200, what is called wicking phenomenonoccurs, by which the solder 202 is sucked up along the bending area γforming the center of elastic deformation of the contact part 102 shownin FIG. 6. In this case, since the solder 202 adheres around the bendingarea γ, the elastic deformation performance of the contact part 102 islowered, and thus it is sometimes impossible to attain elasticdeformation enough for the contact part 102 to be pressed against thegrounding conductor 300 and conduct electricity to the groundingconductor 300.

SUMMARY OF THE INVENTION

Wherefore, an object of the invention is to provide a mounting structureof a ground terminal on a printed circuit board formed by soldering,wherein by solving the problems caused by reflow soldering of thegrounding terminal to the printed circuit board, the printed circuitboard is properly grounded to a grounding conductor.

Another object of the invention is to provide a grounding terminal usedfor the above mounting structure.

In order to achieve the above objects, there is provided a groundingterminal mounted on a printed circuit board which is able to havecontact with a grounding conductor, the grounding terminal comprising: ajoint part having a plurality of joint surfaces which are to be solderedto a conductive pattern on the printed circuit board; a contact partextending from the joint part through a bending area bent in theopposite direction of the joint surfaces, and arranged to be able tohave contact with the grounding conductor, which is elastically deformedupon being pressed by the grounding conductor; and at least one of thecontact surfaces of the joint part having a smaller surface area thanthose of the other contact surfaces.

Also, there is provided a mounting structure in which a groundingterminal able to have contact with a grounding conductor is mounted on aprinted circuit board, the grounding terminal comprising: a joint parthaving a plurality of joint surfaces which are to be soldered to aconductive pattern on the printed circuit board; a contact partextending from the joint part through a bending area bent in theopposite direction of the joint surfaces, and arranged to be able tohave contact with the grounding conductor, which is elastically deformedupon being pressed by the grounding conductor; at least one of thecontact surfaces of the joint part having a smaller surface area thanthose of the other contact surfaces; and the grounding terminal beingmounted on the printed circuit board by reflow soldering, using anamount of solder corresponding to the surface area of each jointsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a plan view showing the mounting structure of a groundingterminal according to the embodiment of the invention;

FIG. 2 is a longitudinal sectional view along line II—II of FIG. 1;

FIGS. 3 and 4 show modifications of the restriction part, respectively,in the invention;

FIG. 5 is a plan view showing a conventional mounting structure of agrounding terminal on a printed circuit board; and

FIG. 6 is a longitudinal sectional view along line VI—VI of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 2, the grounding structure in the embodiment is formedby soldering a grounding terminal 10 to a printed circuit board 20. Thegrounding terminal 10 soldered to the printed circuit board 20 ispressed against a grounding conductor 30, thereby ground the printedcircuit board 20 to the grounding conductor 30. To illustrate thestructure simply, the grounding conductor 30 is not shown in FIG. 1. Thegrounding conductor 30 may be a chassis or a metal shielding case whichcovers the printed circuit board 20.

The grounding terminal 10 in the embodiment is formed by bending astrip-like thin sheet metal member. The longitudinal center part of themetal member is a joint part 11, within which the metal member is bentsuch that the longitudinal section of the metal member forms atrapezoid, in other words, a groove 11 d is formed on one side of thejoint part 11. Two surfaces oppositely positioned across the groove 11 dare joint surfaces 11 a and 11 b. In this case, the groove 11 d isformed at a predetermined distance away from the longitudinal center ofthe joint part 11, and thus one joint surface 11 a is smaller than theother joint surface 11 b.

The metal member is bent in the upper direction at both ends of thejoint part 11. At one end, the metal member is bent approximatelyperpendicularly to the joint part 11, and the bent part is further bentapproximately perpendicularly so as to form a U-shaped longitudinalsection with the joint part 11. Rectangular plate-like parts 14, whichare bent perpendicularly in the direction of the joint part 11, areformed on both sides of the edge of the metal member. Also, projectionparts 15 projecting from both sides of the metal member are formed at apredetermined distance away from the edge. The projection parts 15 aremade by being separated from the neighboring member of the same shapewhen the strip-like thin sheet metal member is stamped out from a sheetmetal

The other end of the metal member is bent at the end of the joint part11 so as to form a V-shaped longitudinal section. The peak of theV-shaped part has a gently curved longitudinal section instead of anacute one. A part which also has a gently curved longitudinal sectionfrom the peak of the V-shape part toward the edge of the metal memberforms, together with the V-shaped part, a contact part 12. The part fromthe end of the contact part 12 to the edge having a straightlongitudinal section is bent in the direction of the joint part 11, andforms a restriction part 13.

Once the grounding terminal 10 is soldered and fixed on the printedcircuit board, which is then placed in a metal shielding case and thelike, the contact part 12 contacts with and is pressed against thegrounding conductor 30, thus being elastically deformed around the peakof the above mentioned V-shaped part.

FIG.2 shows the state that the contact part 12 is elastically deformedby being pressed against the grounding conductor 30. Two-dot chain lineof FIG. 2 shows the state that the contact part 12 is not pressedagainst the grounding conductor 30. The main function of the plate-likeparts 14 is to restrict the movement of the contact part 12 such thatthe elastic deformation of the contact part 12 is performed withoutfluctuation. The restoring force caused by the elastic deformation makesthe contact part 12 securely pressed against the grounding conductor 30,thereby properly grounding the printed circuit board 20 to the groundingconductor 30.

The mounting structure of the invention is formed by soldering,specifically by reflow soldering, the grounding terminal 10 having thejoint part 11, the contact part 12, and the restriction part 13 as abovementioned to the printed circuit board 20.

Specific steps of soldering are explained hereafter.

On the printed circuit board 20 is formed a conductive pattern 21, whichcorresponds to the joint surfaces 11 a and 11 b configured within thejoint part 11 of the grounding terminal 10. Before soldering, creamedsolder 22 a and 22 b are applied on the conductive pattern 21 using aknown metal mask. The grounding terminal 10 is temporarily fixed to aprescribed position on the printed circuit board 20 by taking advantageof adhesion of the creamed solder applied on the conductive pattern 21of the printed circuit board 20. Then the printed circuit board 20, onwhich the grounding terminal 10 is temporarily fixed, is heated in areflow furnace to allow the aforementioned creamed solder 22 to bemelted, and the soldering steps end. The above reflow soldering isautomatically performed, by means of total heating such as infraredheating, latent heating of vaporization, hot air circulation heating,and hot plate heating, and local heating using a heating tool, a lightbeam, a laser beam, and an air heater. Since reflow soldering is known,a detailed explanation of it is omitted.

The aforementioned grounding structure according to the embodiment ofthe invention has particular advantages in the process of reflowsoldering as explained below. To make the explanation easy tounderstand, problems in a conventional grounding structure are firstlyclarified.

In a conventional grounding structure shown in FIG. 6, there are thefollowing problems in soldering the grounding terminal 10 to the printedcircuit board 200 by reflow soldering.

(A) The grounding terminal 100 is moved to the right position byself-alignment effect caused by the melted solder, and then is fixed atthe position by hardening of the melted solder. In this case thegrounding terminal 100 may deviate, during the hardening process, fromthe right position on the printed circuit board 200 because of vibrationand the like. In the case where the grounding terminal deviates from theright position, the contact part 102 cannot be appropriately pressedagainst the grounding conductor 300, and also the requirements of highlyprecise mounting performance to allow high density mounting cannot besatisfied.

(B) When the temperature of the grounding terminal 100 reaches themelting point of the solder earlier than the conductive pattern 201 ofthe printed circuit board 200, what is called wicking phenomenon occurs.Wicking phenomenon, here, means a phenomenon that the solder 202 issucked up along the bending area γ forming the center of elasticdeformation of the contact part 102 shown in FIG. 6. In this case, sincethe solder 202 adheres and hardens around the bending area γ, theelastic deformation performance of the contact part 102 is lowered, andthus it is sometimes impossible to attain elastic deformation enough forthe contact part 102 to be pressed against the grounding conductor 300and conduct electricity to the grounding conductor 300.

In contrast, in the grounding structure according to the embodiment ofthe invention, two joint surfaces 11 a and 11 b having different surfaceareas are formed within the joint part 11 of the grounding terminal 10.As a result, the creamed solder 22 a applied on the relatively smalljoint surface 11 a hardens earlier than the creamed solder 22 b appliedon the relatively large joint surface 11 b, when the printed circuitboard 20 is cooled after the grounding terminal 10 is moved to the rightposition on the printed circuit board 20 by self-alignment effect causedby the surface tension of the melted creamed solder 22 a and 22 b.Accordingly, once the creamed solder 22 a applied on the smaller jointsurface 11 a hardens, the grounding terminal 10 does not deviate fromthe right position on the printed circuit board 20 because of vibrationand the like. This reduces the possibility of the grounding terminal 10deviating from the right position on the printed circuit board 20 whilecooling the printed circuit board 20, and thus has solved the problemsmentioned in (A). Namely, the possibility of the contact part 12 notbeing appropriately pressed against the grounding conductor 30 isreduced, and also the requirements of highly precise mountingperformance to allow high density mounting are satisfied.

Another advantage is as follows: in the case where two joint surfaces 11a and 11 b having different surface areas are formed within the jointpart 11 of the grounding terminal 10, when the printed circuit board 20with the grounding terminal 10 at the prescribed position thereon isheated in a reflow furnace, the solder 22 a applied on the relativelysmall joint surface 11 a is melted earlier than the solder 22 b appliedon the relatively large joint surface 11 b. This may result in what iscalled tombstone phenomenon (Manhattan phenomenon), which means, in thiscase, the grounding terminal 10 is made to erect because of the surfacetension of the melted solder. However, in the embodiment of theinvention, the creamed solder 22 a and 22 b, which is a paste mixture ofpowdered solder and a vehicle, is employed. As a result, there is astrong possibility that the adhesion of the creamed solder offsets theforce to detach the joint surface llb from the conductive pattern 21 onthe printed circuit board 20 caused by the surface tension of thepreviously melted solder. Moreover, in the embodiment of the invention,since the size of the joint surfaces of 11 a and 11 b is different, theadhesion of the creamed solder 22 b on the larger joint surface 11 b isprobably stronger than the force to detach the joint surface 11 b causedby the surface tension of the melted creamed solder 22 a on the smallerjoint surface 11 a. Thus tombstone phenomenon is prevented effectively.

A further advantage is as follows: in the mounting structure of theembodiment, the bending area γ is formed at a predetermined distanceaway from the relatively small joint surface 11 a of the joint part 11.As a result, since the creamed solder 22 a applied on the smaller jointsurface 11 a is not sucked up, by wicking phenomenon, to the bendingarea γ of the contact part 12, the solder 22 a does not adhere aroundthe bending area γ. This prevents the elastic deformation performance ofthe contact part 12 from declining because of the solder adhering aroundthe bending area γ. Thus, the aforementioned problem (B) is solved andthe contact part 12 is appropriately pressed against the groundingconductor 30.

The mounting structure of the embodiment provides another advantage asfollows: a restriction part 13 adjacent to the contact part 12 of thegrounding terminal 10 is formed so that the stress caused to the contactpart 12 by the elastic deformation thereof will not exceed the elasticlimit. When the contact part 12 is elastically deformed to apredetermined extent, the restriction part 13 restricts the elasticdeformation by contacting and pressing its edge to a surface 11 c on thereverse side of the joint surface 11 b of the joint part 11. Thepredetermined extent means the extent of elastic deformation which doesnot allow the stress caused to the contact part 12 to exceed the elasticlimit. This prevents a permanent deformation of the contact part 12 eventhough a heavy load is applied thereto. Although the restriction part 13is formed adjacent to the contact part 12 of the grounding terminal 10in FIG. 2, it may be possible to form a restriction part on the reverseside of the joint surfaces 11 a and 11 b of the joint part 11 as shownin FIG. 3 and 4 such that the restriction part is pressed against thecontact part 12. In a modification shown in FIG. 3, a member 16 isformed as part of the metal member such that the member 16 extends froma side end of the top surface of the above mentioned trapezoid part andbent in the upper direction, allowing the top edge of the member 16 tofunction as a restriction part. In a modification shown in FIG. 4, thetrapezoid part of FIG. 2 is redesigned to have a significant elevation,and the trapezoid part 17 functions as a restriction part.

As described above, the mounting structure of the embodiment have solvedthe problems in soldering the grounding terminal 10 to the printedcircuit board 20 by reflow soldering, ensuring that the printed circuitboard 20 is properly grounded to the grounding conductor 30.

The invention is not restricted to the embodiment and may be embodied invarious forms without departing from the spirit and the scope of theinvention.

For example, in the mounting structure of the embodiment, two jointsurfaces 11 a and 11 b having different surface areas, respectively, areformed within the joint part 11 of the grounding terminal 10. However,when a grounding terminal having a joint part, within which three ormore joint surfaces are formed, is soldered to a printed circuit board,only one of the joint surfaces needs to be smaller than the others.

Also, in the mounting structure of the embodiment, the creamed solder 22a and 22 b is applied to both of the two joint surfaces 11 a and 11 bformed within the joint part 11 of the grounding terminal 10. However,in order to prevent tombstone phenomenon, only the solder applied on thelarger joint surface 11 b needs to be creamed solder 22 b. In otherwords, to prevent tombstone phenomenon, the adhesion on the larger jointsurface 11 b is necessary to resist against the surface tension of thesolder which is melted first on the smaller joint surface 11 a.

What is claimed is:
 1. A grounding terminal for mounting to a printedcircuit board and engagement with a grounding conductor, the groundingterminal comprising: a joint part having a plurality of joint surfacesfor soldering the grounding terminal to a conductive pattern on aprinted circuit board; a contact part extending from and being connectedto the joint part via a bending part, the contact part bending back overthe joint surfaces and being configured to facilitate engagement with agrounding conductor, and the contact part being elastically deformabletoward a rear surface of the joint surfaces upon engaging with agrounding conductor; and at least one of the joint surfaces of the jointpart having a surface area which is smaller than a surface area of allremaining joint surfaces; wherein there is difference in hardeningperiods of solder applied to the joint surfaces between the smallerjoint surface and the remaining joint surfaces of the joint part.
 2. Thegrounding terminal according to claim 1, wherein the grounding terminalfurther comprises at least one intermediate part between the smallersurface and the remaining surfaces, the intermediate part is trapezoidalso that the intermediate part, once the contact part is deformed to apredetermined extent, contacts with the contact part to restrict furtherelastic deformation of the contact part.
 3. The grounding terminalaccording to claim 1, wherein the bending part is formed a predetermineddistance away from the joint surface of the joint part.